This invention relates to conveyor ovens having reduced fuel consumption and with quieter operation, and more particularly to such ovens having an energy management system including a modulated gas flow with a safety feature that isolates electrical controls in order to enhance an integrity of the safety features built into the oven.
Prior art conveyor ovens are shown in U.S. Pat. Nos. 4,964,392 and 5,277,105 owned by the assignee of this invention and in the references cited on the cover pages of these patents. These and other similar patents may be consulted in order to learn details of how conveyor ovens are constructed and operate. Oftenxe2x80x94but not alwaysxe2x80x94this type of oven is used to cook or bake pizzas, bread, or the like.
Conveyor ovens are devices for automatically baking or cooking food products over timed periods. Normally, they have a conveyor belt which travels through an elongated oven cavity having open ends and at a speed which times the exposure of the food product to the heat of the oven. A food product, such as a pizza, for example, is placed on one end of the conveyor at the entry to the oven cavity and delivered from the oven at the opposite end of the cavity. The heat in the oven and the speed of the conveyor are coordinated so that the food product is fully and correctly cooked or baked by the time when the conveyor delivers it at the exit end.
The conventional method of delivering controlled heat has been to switch burners off and on in order to hold the resulting temperature in the oven cavity within a relatively narrow range. This process has functioned very well in the past. However, anything can always be improved and, therefore, it is always possible to do a better job heating and cooking or baking the food product.
Also, the cost of the fuel (natural or propane gas) for the burners is increasing sharply. Thus, an important goal is to reduce the fuel consumption, which the invention has done by approximately 30%.
A conveyor oven is usually energized by gas, which always requires a safety feature since an explosive atmosphere might be created by leaking gas. As a result, various governmental agencies prescribe an incorporation of safety features, such as a cut-off valve, into the design of the oven. It is important for the oven controls to be designed to not only enable a reliability of these safety features, but also to provide a redundancy for the safety features. In this invention, the safety features are enhanced by providing an electrical isolation between control signals that might otherwise cause a feed back that might enable the oven to continue operation or to re-ignite after a demand for shut down.
Accordingly, an object of the invention is to cook or bake a better food product by maintaining a closer control over the uniformity of the heat in the oven cavity. Here, an object is to maintain a substantially smooth level of heat after the oven is switched on and continuing throughout the oven operation. In particular, an object is to avoid the peaks and valleys of heat swings as the burner switches on and off as it hunts for the targeted temperature.
Another object is to provide a quieter operation by eliminating a blower-like noise which occurred heretofore as the burner switched on.
Still another object is to provide a universal heat controller which can control either a modulating valve or an on/off valve, thereby eliminating a need for many controllers individually dedicated to specific ovens.
A further object of the invention is to provide a control circuit which insures the integrity of a safety feature built into the oven. Here, an object is to provide a self-healing control which so insures the integrity, but which does not require a replacement of a fuse or other element after the condition requiring a safety valve operation subsides.
In keeping with an aspect of the invention, these and other objects are accomplished by a use of a modulating valve which increases or decreases the amount of the gas flow to a burner without fully switching the burner off or on during a bake cycle. The modulating valve is controlled responsive to temperatures sensed by thermocouple sensors located in the oven. Furthermore, the controller is also able to control an on/off gas valve in response to the same type of sensor signals, so that the same controller may be used universally for both the modern oven using the modulating valve and the older ovens using on/off valves.
The integrity of the safety features built into the oven is preserved by an isolation in the electrical control circuit between the elements which control the modulating valve and a conditioner which converts signals from sensors into signals which can enable an operation of the modulating valve. This isolation prevents feed back which might otherwise cause the modulating valve to behave in a way which would defeat the shutdown of an on/off safety valve. In order to provide a self-healing of this integrity insuring system, it uses transformers for separating and isolating the modulation valve control and conditioner circuits from an ignition control module.
The advantages of the invention are many. There is an improved reliability and a higher quality bake at a shorter bake time and at a lower average temperature. Since the ovens operate at a lower temperature, the components are in a cooler environment which extends their life. The oven is quieter and the energy management system is more efficient due to a use of the modulating gas valve and to a two-way air return, with less turbulence, creating lower DB noise levels. There is an increased flexibility making it easier to rearrange the fingers for delivering heated air to the food product. There is an ability to add a deck, as volume increases, or to remove a deck if volume falls off. All decks are the same.